Re: [PATCH] mm: yield during swap prefetching

[Andrew Morton]

Con Kolivas <kernel@xxxxxxxxxxx> wrote:
>
> Swap prefetching doesn't use very much cpu but spends a lot of time waiting on 
> disk in uninterruptible sleep. This means it won't get preempted often even at 
> a low nice level since it is seen as sleeping most of the time. We want to 
> minimise its cpu impact so yield where possible.
> 
> Signed-off-by: Con Kolivas <kernel@xxxxxxxxxxx>
> ---
>  mm/swap_prefetch.c |    1 +
>  1 file changed, 1 insertion(+)
> 
> Index: linux-2.6.15-ck5/mm/swap_prefetch.c
> ===================================================================
> --- linux-2.6.15-ck5.orig/mm/swap_prefetch.c	2006-03-02 14:00:46.000000000 +1100
> +++ linux-2.6.15-ck5/mm/swap_prefetch.c	2006-03-08 08:49:32.000000000 +1100
> @@ -421,6 +421,7 @@ static enum trickle_return trickle_swap(
>  
>  		if (trickle_swap_cache_async(swp_entry, node) == TRICKLE_DELAY)
>  			break;
> +		yield();
>  	}
>  
>  	if (sp_stat.prefetched_pages) {

yield() really sucks if there are a lot of runnable tasks.  And the amount
of CPU which that thread uses isn't likely to matter anyway.

I think it'd be better to just not do this.  Perhaps alter the thread's
static priority instead?  Does the scheduler have a knob which can be used
to disable a tasks's dynamic priority boost heuristic?
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